Undercut thread closure

ABSTRACT

A thread system for use on screw type closure structures for high pressure vessels and the like wherein a helical fillet blends the undercut of the vessel wall to the first thread, thereby lessening stress concentrations in the vessel wall. The fillet is shaped such that at all points around the helical path described by the leading surface of the first thread, the transition from that surface into the undercut is by means of the desired fillet. In some embodiments the fillet is in the form of a quarter of an ellipse.

United States Patent Witkin 1451 June 13, 1972 3,425,588 2/1969 Mathews 1., ..220/39 Primary Examiner-M. Henson Wood, Jr.

Assistant Examiner-Michael Y. Mar

Attorney-Charles B. Smith ABSTRACT A thread system for use on screw type closure structures for high pressure vessels and the like wherein a helical fillet blends the undercut of the vessel wall to the first thread, thereby lessening stress concentrations in the vessel wall. The fillet is shaped such that at all points around the helical path described by the leading surface of the first thread, the transition from that surface into the undercut is by means of the desired fillet. In some embodiments the fillet is in the form of a quarter of an ellipse.

8 Claim, 5 Drawing Figures BACKGROUND OF THE INVENTION This invention relates to an improvement in closure means of the threaded type for metallic vessels utilized in high pressure applications and more particularly to an improved configuration of the vessel thread to undercut jointure which minimizes stresses in the area of the pressure vessel closure when under high pressure.

In pressure vessel systems using threaded closures, it is wellknown that when a thread is terminated with a feathered edge, extremely high local stresses result. Thus the usual procedure in highly stressed threaded closures is to remove the feathered edge so that only full thread sections are used. The thread is first cut, then the undercut is machined with a large radial shaped fillet blended to the first thread in order to lessen stress concentrations. This machined fillet has the same radial configuration at any point along a circumferential line created by the intersection of a plane perpendicular to the axis of the pressure vessel, with the inside of the pressure vessel wall.

While this radial fillet provides for lesser stress concentrations than no fillet at all, nevertheless large stress concentrations still result. This necessitates overdesigning the cross section of the vessel wall in order to provide adequate safety factors. It is often the case that during the removal of the feathered edge of the thread, the undercut fillet is also removed at all points along the thread other than at the beginning of the thread. This adds to the concentration of high local stresses in the undercut to thread juncture.

The principal object of the invention is to provide a novel and improved threaded closure for a pressure vessel wherein the stress concentrations in the vicinity of the threaded portion of the vessel wall are minimized.

SUMMARY OF THE INVENTION The invention overcomes these and other problems by providing a helical fillet between the undercut and the first load bearing thread. The profile of the fillet follows the helix described by the thread form, so that at all points around the helical path described by the leading surface of the first thread, that surface blends into the undercut by means of the fillet. In some embodiments the fillet is in the form of substantially a quarter of an ellipse. In one particularly advantageous embodiment the ratio of the major to minor diameters of the elliptical form is four to one.

The design of the invention distributes the stresses in the thread undercut portion of the closure resulting when the vessel is pressurized, so that stress concentration is minimized. This allows a thinner vessel wall design than most prior pressure vessels of the threaded closure type.

In still other embodiments the elliptical fillet does not follow the helical thread form but has the same configuration at any point along a circumferential line created by the intersection of a plane, perpendicular to the axis of the pressure vessel, with the inside of the pressure vessel wall.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in greater detail with reference to the appended drawings in which:

FIG. 1 is a view in central vertical section of the closure area of a high pressure vessel;

FIG. 2 is a diagrammatic view in central vertical section of a portion of the pressure vessel thread undercut of FIG. 1;

FIG. 3 is a perspective view taken generally along the line 3-3 of FIG. 2 showing a portion of the elliptical fillet of the invention after the initial machining;

FIG. 4 is a view similar to FIG. 3 showing the fillet after further refinements; and

FIG. 5 is a graph comparing the distribution of an applied load on the vessel wall of a conventional thread closure and on the vessel wall of the invention.

2 DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference now to FIGS. 1 and 2 in particular, the high pressure vessel 10 may have any configuration and only a portion thereof provided with a threaded opening 12 into which a threaded plug 14 is screwed to close off the interior of the vessel has been illustrated.

The vessel wall 16 has an undercut l8 beneath the first or innermost thread 20. The portion of the vessel wall 16 which joins the undercut 18 to the first thread 20 is designated 22 and has a configuration in the form of a quarter of an ellipse, as is illustrated more clearly in FIG. 2. The greatest rate of curvature of this elliptical fillet 22 occurs adjacent the first thread 20. The fillet 22 then tapers downwardly with a decreasing rate of curvature to blend with the main vessel wall 16. The fillet 22 is of a different geometric shape than that used between the adjacent threads.

An elliptical fillet of any such configuration provides for lesser concentration of stresses in the vicinity of the first thread when the vessel is pressurized than a fillet of radial configuration. In the preferred embodiments of the invention the ratio of the major diameter, a, of the elliptical fillet 22 to the minor elliptical diameter, b, is four to one. Other embodiments could utilize different ratios depending on the peculiarities of the overall pressure vessel design, and could be greater or less than a quarter of an ellipse.

In some embodiments, such as in vessel closures having discontinuous or lug type threads, the elliptical fillet has the same configuration at any point beneath the thread form along a circumferential line created by the intersection of a plane, perpendicular to the central axis 24 of the pressure vessel, with the inside of the vessel wall 16. The stress concentration of this shape is less than in a symmetrically radial fillet.

In a preferred embodiment, the fillet 22 is not symmetrical about the central axis 24 but is generated such that its profile follows the helix described by the form of the first thread 20. Thus at all points around the helical path described by the leading surface of the first thread 20, that surface blends into the undercut 18 by means of the desired elliptical fillet 22.

Usually this operation is performed on a lathe using the same lead screw arrangement as is used to machine ,the threaded opening 12. The operation is completed by handworking those areas where it is not possible to insert the cutting tool because of lack of clearance or runout. However, other means of manufacturing this configuration can be used to create the design of the invention.

In FIG. 3, a portion of the fillet 22 is shown after the lathe operation described above, but prior to any handworking. In FIG. 4 the same'portion is shown with the excess material designated as 26 left by the lathe operation removed by handworking. A wedge shaped portion 28 of the terminus of the first thread is removed to make this part of the thread more flexible and to prevent it from being overloaded.

This helical, elliptical fillet embodiment is most efiective in distributing stress and with a four to one ratio of its major diameter to its minor diameter it is still more effective in distributing stress. In other embodiments, however, the helical fillet is of other geometric shapes including radial fillets, parabolic fillets or other conic-section shaped fillets. In some embodiments the shape of the helical fillet may be determined by exponential functions.

As is illustrated in FIG. 5 by the curve 30, the stress in the vessel wall 16 in the vicinity of the first thread 20 becomes highly concentrated in a vessel with a conventional closure. The helical, elliptical fillet 22 lessens the concentration of stress by more effectively distributing it as is illustrated by the curve 32.

The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described, or portions thereof, it being recognized that various modifications are,possible within the scope of the invention.

What is claimed is:

1. An improved screw type closure structure for high pressure vessels and the like of the type wherein a male threaded member and a female threaded member are screwed into complementary threaded engagement, the threads being of helical form and at least one of the two members being undercut from the first load bearing thread, wherein the improvement comprises a fillet which blends the undercut to the first load bearing thread, the fillet being shaped such that at substantially all points around the helical path described by the leading surface of the first load bearing thread, the transition from that surface into the undercut is by means of the fillet.

2. An improved screw type closure structure for high pressure vessels as recited in claim 1 wherein the fillet is of a predetermined geometric shape having its greatest rate of curvature adjacent the first load bearing thread.

3. An improved screw type closure structure for high pressure vessels as recited in claim 1 wherein the fillet is in the shape of a portion of a conic section.

4. An improved screw type closure structure for high pressure vessels and the like of the type wherein a male threaded member and a female threaded member are screwed into complementary threaded engagement, the threads being of helical form and at least one of the two members being undercut from the first load bearing thread, wherein the improvement comprises a fillet in the form of substantially a quarter of an ellipse which blends the undercut to the first load bearing thread, the greatest rate of curvature of the fillet being adjacent the first load bearing thread.

5. An improved screw type closure structure for high pressure vessels as recited in claim 4 wherein the elliptical fillet is shaped such that at substantially all points around the helical path described by the leading surface of the first load bearing thread, the transition from that surface into the undercut is by means of the elliptical fillet.

6. An improved screw type closure structure for high pressure vessels as recited in claim 4 wherein the ratio of the major diameter to the minor diameter of the elliptical form of the fillet is four to one.

7. An improved screw type closure structure for high pressure vessels and the like of the type wherein a male threaded member and a female threaded member are screwed into complementary threaded engagement, at least one of the two members being undercut from the first load bearing thread, wherein the improvement comprises a fillet in the form of substantially a quarter of an ellipse which blends the undercut to the first load bearing thread, the greatest rate of curvature of the fillet being adjacent the first load bearing thread.

8. An improved screw type closure structure for high pressure vessels as recited in claim 7 wherein the ratio of the major diameter to the minor diameter of the elliptical form of the fillet is four to one. 

1. An improved screw type closure structure for high pressure vessels and the like of the type wherein a male threaded member and a female threaded member are screwed into complementary threaded engagement, the threads being of helical form and at least one of the two members being undercut from the first load bearing thread, wherein the improvement comprises a fillet which blends the undercut to the first load bearing thread, the fillet being shaped such that at substantially all points around the helical path described by the leading surface of the first load bearing thread, the transition from that surface into the undercut is by means of the fillet.
 2. An improved screw type closure structure for high pressure vessels as recited in claim 1 wherein the fillet is of a predetermined geometric shape having its greatest rate of curvature adjacent the first load bearing thread.
 3. An improved screw type closure structure for high pressure vessels as recited in claim 1 wherein the fillet is in the shape of a portion of a conic section.
 4. An improved screw type closure structure for high pressure vessels and the like of the type wherein a male threaded member and a female threaded member are screwed into complementary threaded engagement, the threads being of helical form and at least one of the two members being undercut from the first load bearing thread, wherein the improvement comprises a fillet in the form of substantially a quarter of an ellipse which blends the undercut to the first load bearing thread, the greatest rate of curvature of the fillet being adjacent the first load bearing thread.
 5. An improved screw type closure structure for high pressure vessels as recited in claim 4 wherein the elliptical fillet is shaped such that at substantially all points around the helical path described by the leading surface of the first load bearing thread, the transition from that surface into the undercut is by means of the elliptical fillet.
 6. An improved screw type closure structure for high pressure vessels as recited in claim 4 wherein the ratio of the major diameter to the minor diameter of the elliptical form of the fillet is four to one.
 7. An improved screw type closure structure for high pressure vessels and the like of the type wherein a male threaded member and a female threaded member are screwed into complementary threaded engagement, at least one of the two members being undercut from the first load bearing thread, wherein the improvement comprises a fillet in the form of substantially a quarter of an ellipse which blends the undercut to the first load bearing thread, the greatest rate of curvature of the fillet being adjacent the first load bearing thread.
 8. An improved screw type closure structure for high pressure vessels as recited in claim 7 wherein the ratio of the major diameter to the minor diameter of the elliptIcal form of the fillet is four to one. 